Photometer



Aug. 1s, l1936.

J. A. GREENWOOD PHOTOMETER Filed Aug.'26,

Patented Aug. 18, 1936 @siren STATES PATENT orifice 4 Claims.

This invention relates to photometers.

An object of the invention is the provision of a device which may be used independently of a camera or may be employed in the camera for determining the value of the light entering the camera from an object by a comparison with a source of `illumination whose value is known.

Another object of the invention is the provision of a device for comparing an unknown source of light with a known fixed source so that it is possible by such comparison to determine in advance the proper timing of the operation of a diaphragm of a camera and also the proper area of the light opening of the camera as controlled by the movement of the elements composing the diaphragm.

A further object of the invention is the provision ofv a device for comparing an unknown source of light with a known fixed source such` as the constant source of light produced by a highly luminescent radium compound, the compound being disposed in close association with a ground glass plate having substantially the same color as the luminescent compound with the unknown source of light passing t' rough the ground glass, an opening through which the unknown source of light passes being enlarged or decreased in area until the colored ground glass discloses substantially the same luminosity as the glowing radium compound.

A still further object of the invention is the provision of a device having an opening through which an unknown source of light passes with means for controlling the enlargement of the opening until a colored ground glass in close associaton with a radium compound of known .light intensity corresponds in luminosity to that of the radium compound,- the diaphragm controlling the opening in the device being provided with scales indicating the periods of time that the radium compound has been in use with the scales being constructively associated with a cable within convenient reach of the operator so that Cil it is possible to determine the time for the exposure for producing aphotograph and the area lof the ,opening of the diaphragm of the camera for that particular timing of the opening of the diaphragm.

This invention will be best understood from a consideration of the following detailed descrip` tion, in .view of the accompanying drawing forming a part of the specification; nevertheless, it is to be understood that the invention is not conn'ed to the disclosure, being susceptible of such changes and modiiications as define no material -departure from the salient features of the invention as expressed in the appended claims.

In the drawing:

Figure 1 is a vie'w in perspective of a photometer constructed in accordance with .the prlnciples of my invention,

Figure 2 is a longitudinal vertical section taken along the line 2--2 of Figure 1,

Figure 3 is a transverse sectional view taken along the line 3-3 of Figure 2,

lFigure 4 is a fragmentary. horizontal section taken along the line 4 4 of Figure 2,

Figure 5 is a transverse vertical section taken along the line 5 5 of Figure 2, Y

Figure 6 is a view in perspective of one element l5 of a diaphragm, and

Figure 'I is a view in perspective of a holder for a casing for radiating compound showing the casing displaced from -the holder.

Referring more particularly to l0 designates generally a housing having a bottom Il, end'walls I2 and I3, a top H and side I walls I5 and I6.

Intermediate the ends of the top I4 there is provided an opening which gives 'access to the 25 interior of the housing at this point. This opening is closed by a door 2l hinged at 22 to the side wall I5. This door is neatly tted in the opening to. exclude light from the interior of the housing IB.

The end wall I3 is provided with an opening which is covered by a clear glass plate 26 to exclude dust or other foreign matter from the interior of the housing.' The plate 26 is secured in position in any approved manner and normally rests upon a block 21 secured to the bottom H of the housing. l

A ground glass 28 is spaced from the glass 26 and has its lower outer face in engagement with the block 21. This ground glass is secured in position in any approved manner and cooperates with the plate 28 to provide a closed chamber 29 at the forward end of the housing.

A pair of slidably mounted plates 30 and 3l are located in this chamber and are formed of opaque material.y The inner ends of these plates iit in close association with each other and have `their lower ends mounted in a groove 32 in the is also provided with a rack 38 which is adapted 55 the drawing, 20

' to be engaged by the gear 35 and these- .racks each other when the members 30, 3| are moved,

and since the side edges of these openings on each plate are at right angles to each other the openings through which the light is transmitted in this diaphragm will always be rectangular or more particularlyvin the form of a square with the sides of the opening always equal to each other and this is true regardless of the fact of whether the opening has been increased or decreased in area.

The V- shaped opening 4| of the member 3| is provided along its' edge 42 with a scale 43 and this scale is divided into a number of graduations. The adjacent edge 43a of. the same opening 4| is provided with a. scalel 44 which is divided into a plurality of graduations. The V-shaped opening 40 has one edge 46a divided into a plurality of graduations, as shown at 46, while the adjacent edge 41 is divided into a plurality of graduations as shown at 48. The numerals 0 to 16 on these scales refer to four series of periods and represent months elapsed since the insertion of the radium compound button, as will be presently explained. The ground glass is secured within the housing l0 in spaced parallel relation with the ground glass 28.

A third ground glass 5| is slidably mounted in grooves 52 formed in the side walls l5 and I6 of the housing I0 so that the ground glass may be removed when desired and for the purpose which will be presently explained. In order to remove this ground glass the door 2| must be raised for the purpose.

A holder 53 in the form of a plate is removably positioned `within the. housing l0 and in close association with the ground glass 5|. 'Ihe central portion of the plate 53 is provided with a. ground glass of a particular color as shown at 54. The ground glass 54 is in the same plane with the plate 53 and it is in line with the opening provided by the diaphragm at the frontend of the housing so that it will be affected by the light from an object. The value of this light from the object is unknown and is to be deter-y mined.

An opaque'disc 55 is'secured lto the central portion of the ground glass 53 and is adapted to shield a luminescent radium compound enclosed within a transparent casing 56 from the light which enters the opening in the diaphragm. The casing 56, as shown more particularly in Figs. 2, 4, and 7, is secured to the ground glass disc 54 by means of wire loops 51 carried by the disc v54.

An eye piece 60 is mounted in an opening 6I in the rear wall `|2 of the housing I0 and has its outer Vencl preferably in the shape of an`oval so that it will neatly fit the eye of the operator and prevent light at that point from affecting the interior of the housing. A lens 62 is mounted at the inner end of the eye piece and has lts focal point approximately at the casing 56.

Since the luminescent radium compound progressively loses its light-giving power all deduc cated on the card 66.

housing I0 and the opening 25 is directedl to- V26, 28, 50, 5I and the disc 54.

tions which are made for determining the value` of the unknown source of light must be revised in accordance with the age of the radium compound and in order to do this it `is. necesary for the operator to have beforey him the date on 5 'which the compound was first used or purchased.

For this purpose a tag holder. shown at 65 is secured to the side walls Hi and is adapted'to receive a card 66 on which is placed lthe date of the purchase or the placementof the radium .com-` pound in the housing l0.

Permanent indicia is applied to the outer face of the side wall I6 of the housing and includes in the first horizontal column the numerals. one to ten, inclusive of an arbitrary scale, or these numlerals may be increased as may be necessary. It

.=of numerals at one end of a corresponding horizontal row. The numerals in this first column indicate the fractions of the second for the exposures. during snapshots, except the last two figures. 61, and which are in corresponding rows with certain numerals in the first vertical column 69 represent the degree of openings of the diaphragm of the camera in accordance with the time elapsed from the date shown on the card 66. 35

In the horizontal columns, indicated by the numerals 1n and 1I, are placed indicia'for time exposures and the corresponding diaphragm openings for the various periods after the time indi- When the operator desires to take a photograph he places the member .60 in enclosing relation with his eye and operates the knob 39 for opening the diaphragmat the forward end of the wards the object to be photographed. `Light coming from the object will pass through the plates The transmitted light through the disc 54 may have a greater or less value than the light from the casing 56. Whatever the case may be, however, the knob 39 is adjusted until the disc 54 shows approximately the same intensity of light as the radium cornpound in the casing 56.

When time exposures are taken it may be necessary to' replace the plate 5| by the plate which is less opaque and this can be done readily by raising the door 2| and removing the plate 5I from the grooves 52.

The scales on the diaphragm members 30 and 60 3| indicate the age limits of the radium compound and it will be noted that there are four dif-. ferently calibrated scales which effectively quadruple the usefulness or longevity of the radium compoundin the casing 56. When the radium 65 compound has been sufficiently dissipated it is discarded and a new casing with a fresh com pound is placed upon thedisc 54and supported by the holder 51.

Upon looking through the eye piece 60 with 70 the instrument pointed at the object to be photographed the micrometer screw 39 is turned andw.-

the opening of the diaphragm members 3| and 30 is such that the field of light from the object blends with the luminescent source in the casing The numerals one to 20 The numerals in the vertical columns 30A corresponding times for exposure.

aosta 56. Upon the proper one of the four scales 53, M, o6, and itl' as determined by the time elapsed from the date on the card te, the nearest number of marl-rs yon the edge of the square are counted. On the table shown on the side wall i6 the number from the scales previously mentioned is found in the rst row te and this may be three. Foilowing down the third column we come tothe horizontal row marked te and we find the numeral twenty-two which gives the opening in the diaphragm While the numeral twenty-live in the first vertical row gives the fractionof a second for the snapshot exposed. l

The number to be selected in the horizontal row 68 is found on one of the scales S3 to M inclusive by observing the point on one scale where it is cut by the overlapping edge oi another scale.

In other words where the time elapsed is between 4 and 8 months the scale it is used and the numeral selected in the horizontal row 68' is determined on the scale it where the edge 66a cuts the scale Mi.

The scales t3 to lil' inclusive are determined in advance and experimently in the laboratory The series of numerals -4, 4-8, 8-12, and 12-16 serve the purpose of identifying the scales to be used in accordance with the time elapsed since the application of the radium compound. The scales, however, as indicated by the line-markings and numerals as cut by a sliding edge of a section of the diaphragm select or determine which one of the columns El is to be employed to give the arbitrary numerical value of the diaphragm opening. The numeral in a horizontal column @t selected by the column 6l andthe rst vertical column gives the corresponding speed for snapshots.

The scales tt to il inclusive are divided in accordance with experimental computations and therefore there will be variations, between the units on the different scales. If for a given stopopening and length of exposure one of the unit markings is knownyfall the other markings on that scale may be accurately determined.

In talring time exposures the length of exposure is important. In rows it and 'ii two stop-opening selections are illustrated with their 'I'hus if .on scale el three marks are shown while using the time exposure plate 5i, then with opening f7.9, the iilm should be exposed eight-tenths oi a second. It is noted that the number of marks read on the correct one of the four scales it to 'i inclusive determines which of the columns is to be used. The operator has a choice of any one of the numbers in that column. All the combinations in the column 6l particularly in the snap-shot group admit the same total amount of light.

I claim:

l. A photometer comprising a housing having a light-transmitting opening at one end and an eye piece at the opposite end in line with the opening, a luminescent radium compound disposed in line between the-light-transmitting opening and the eye piece, a plate, a lens in the eye-piece and having its focal point at the compound, a translucent disc disposed centrally of the plate and in contact with the compound, an opaque shield between the compound and the disc, means for removably securing the shield and compound on the disc, a ground -glass plate slidably mounted in the casing and in contact with the first-mentioned the light-transmitting opening so that the disc through which light is transmitted will ve the same light value as the luminescent means, the diaphragm including members movable relative to each other, said members having il-shaped notches and each edge of the notches being provided with a section of a scale having graduations representing periods of time that the luminescent means has been in use, corresponding edges of the v notches moving with respect to each other when said diaphragm members are moved over each other to vary the light opening, one of the V- notch edges denoting on `the scale of another V- notch edge a graduation' which determines, the data to be selected from indicia on the housing for determining the diaphragm opening of a camera and the time exposure for a nlm.

3. A photometer comprising a housing having a light transmitting opening and a sight opening in line with each other, the housing being provided with a door intermediate the ends thereof,

a plate mounted in the housing adjacent thedoor 3 having a centrally disposed translucent disc, a luminescent means removably supported centrally of the disc, a mask between the luminescent means and the disc, a second plate in contact with the first plate for diffusing light from the light transmitting opening, means for slidaloly mounting the second plate in' the housing adjacent the door so that said plate may be removed from the housing, and means for controlling the quantity. of light admitted through the light-transmitting opening so that the disc through which light is transmitted through the second plate and the disc will have approximately the se light value as the luminescent means.

i. A photometer comprising a housing having a light-transmitting opening and a sight opening" in alignment, a constantly diminishing source of light between the two'openings, a diaphragm at the light-transmitting opening composed of a' pair of movable members having il-shaped notches at the inner adjacent edges, each of the overlapping edges having a section ot a scale, the sections being composed of graduations representing periods of time that the source of light has been in use, the overlapping edges being movable over each other to provide a square opening of an area. that the light admitted through the first opening will have the approximate value of the source of light which has diminished in proportion to the time period it has been used, the section of the scale representing such time period being cut by a cooperating sliding edge and indieating a graduation which corresponds to a compensating scale on the housing representing the diaphragm opening of a. camera and time exposure for a nlm for that particular iight value.

JOSEPH A. GREENWOOD. 

